Messages in this thread | | | Date | Fri, 12 Apr 2013 16:47:50 -0700 | Subject | Re: [PATCH] module: Fix race condition between load and unload module | From | Linus Torvalds <> |
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On Fri, Apr 12, 2013 at 3:32 PM, Anatol Pomozov <anatol.pomozov@gmail.com> wrote: > > Here is timeline for the crash in case if kset_find_obj() searches for > an object tht nobody holds and other thread is doing kobject_put() > on the same kobject: > > THREAD A (calls kset_find_obj()) THREAD B (calls kobject_put()) > splin_lock() > atomic_dec_return(kobj->kref), counter gets zero here > ... starts kobject cleanup .... > spin_lock() // WAIT thread A in kobj_kset_leave() > iterate over kset->list > atomic_inc(kobj->kref) (counter becomes 1) > spin_unlock() > spin_lock() // taken > // it does not know that thread A increased counter so it > remove obj from list > spin_unlock() > vfree(module) // frees module object with containing kobj > > // kobj points to freed memory area!! > koubject_put(kobj) // OOPS!!!!
This is a much more generic bug in kobjects, and I would hate to add some random workaround for just one case of this bug like you do. The more fundamental bug needs to be fixed too.
I think the more fundamental bugfix is to just fix kobject_get() to return NULL if the refcount was zero, because in that case the kobject no longer really exists.
So instead of having
kref_get(&kobj->kref);
it should do
if (!atomic_inc_not_zero(&kobj->kref.refcount)) kobj = NULL;
and I think that should fix your race automatically, no? Proper patch attached (but TOTALLY UNTESTED - it seems to compile, though).
The problem is that we lose the warning for when the refcount is zero and somebody does a kobject_get(), but that is ok *assuming* that people actually check the return value of kobject_get() rather than just "know" that if they passed in a non-NULL kobj, they'll get it right back.
Greg - please take a look... I'm adding Al to the discussion too, because Al just *loooves* these kinds of races ;)
Linus [unhandled content-type:application/octet-stream] | |